Nuclear Reactor: Definition, Components

– It has been possible to control the fission of U-235 so that energy is released slowly at a usable rate.

– Controlled fission is carried out in a specially designed plant called a nuclear power reactor or simply a nuclear reactor.

Components of Nuclear reactor

– The chief components of a nuclear reactor are:

(1) U-235 fuel rods

– U-235 fuel rods which constitute the (fuel core).

– The fission of U-235 produces heat energy and neutrons that start the chain reaction.

(2) Moderator

– Moderator which slows down or moderates the neutrons

– The most commonly used moderator is ordinary water.

– Graphite rods are sometimes used.

– Neutrons slow down by losing energy due to collisions with atoms/molecules of the moderator.

(3) Control rods

– Control rods which control the rate of fission of U-235.

– These are made of boron-10 or cadmium, which absorbs some of the slowed neutrons.

– Thus the chain reaction is prevented from going too fast.

(4) Coolant

– Coolant which cools the fuel core by removing heat produced by fission.

– Water used in the reactor serves both as a moderator and coolant.

– Heavy water (D₂O) is even more efficient than light water.

(5) Concrete shield

– Concrete shield that protects the operating personnel and environments from destruction in case of leakage of radiation

Light-water Nuclear power plant

– Most commercial power plants today are (light-water reactors).

– In this type of reactor, U 235 fuel rods are submerged in water.

– Here, water acts as a coolant and moderator.

– The control rods of boron-10 are inserted or removed automatically from spaces in between the fuel rods.

– The heat emitted by the fission of U-235 in the fuel core is absorbed by the coolant.

– The heated coolant (water at 300°C) then goes to the exchanger.

– Here the coolant transfers heat to sea water which is converted into steam.

– The steam then turns the turbines, generating electricity.

– A reactor once started can continue to function and supply power for generations.

– About 15 percent of consumable electricity in the U.S.A. today is provided by light water reactors.

– India’s first nuclear plant went into operation in 1960 at Tarapur near Mumbai.

– Another plant has been set up at Narora in Uttar Pradesh.

While such nuclear power plants will be a boon for our country, they could pose a danger to the environment.

– In May 1986, the leakage of radioactive material from the Chornobyl nuclear plant in the USSR played havoc with life and property around.

– The disposal of reactor waste poses another hazard.

– The products of fission e.g., Ba-139 and Kr 92, are themselves radioactive.

– They emit dangerous radiation for several hundred years.

– The waste is packed in concrete barrels which are buried deep in the earth or dumped in the sea.

– However, the fear is that any leakage and corrosion of the storage vessels may eventually contaminate the water supplies.

Breeder Reactor

– We have seen that uranium-235 is used as a reactor fuel for producing electricity.

– But our limited supplies of uranium-235 are predicted to last only for another fifty years.

– However, nonfissionable uranium-238 is about 100 times more plentiful in nature.

– This is used as a source of energy in the so-called breeder reactors which can supply energy to the world for 5,000 years or more.

– Here the uranium-235 core is covered with a layer or ‘blanket’ of uranium-238.

– The neutrons released by the core are absorbed by the blanket of uranium-238.

– This is then converted to fissionable plutonium-239.

– It undergoes a chain reaction, producing more neutrons and energy.

– The above reaction sequence produces three neutrons and consumes only two.

– The excess neutron goes to convert more uranium to plutonium-239.

– Thus the reactor produces or ‘breeds’ its own fuel hence its name.

– Several breeder reactors are now functioning in Europe.

– However, there is opposition to these reactors because the plutonium so obtained can be used in the dreaded H-bomb.